1. Field of the Invention
The present invention relates to a process for preparing aniline by hydrogenating nitrobenzene. Aniline is a very important compound as a basic raw material for urethanes, rubber chemicals, dyes, medicines and the like.
2. Description of the Related Art
U.S. Pat. No. 2,292,879 discloses a process in which an aromatic nitro-compound is hydrogenated in a liquid phase to prepare a corresponding aromatic amine. In this process, a catalyst of nickel, cobalt or copper supported on a finely ground carrier is dispersed in the mixture of an aromatic nitro-compound and an aromatic amine which is the hydrogenation product of the nitro-compound. Hydrogen is then passed through the mixture to carry out reaction. The specification of the U.S. patent discloses that the reaction is carried out under conditions of continuously distilling off water formed by the reaction together with the amine from the reaction system, whereby the activity of the catalyst can be maintained at a high level, and that the aromatic amine is used as a reaction solvent in the reaction so as to relatively heighten the concentration of this amine, whereby the activity of the catalyst can be increased.
Japanese Patent Publication No. 50-15779 discloses that in a liquid phase reaction as in the above-mentioned U.S. patent, the concentration of a produced aromatic amine which is used as a solvent is maintained at 95% by weight or more in the liquid phase, and the reaction is carried out under a pressure as close as possible to atmospheric pressure and at the boiling point of the liquid or a temperature close thereto, so that water produced by the reaction can be easily removed from the system and so that the aromatic amine of the product distilled off from the reactor conveniently contains a lower amount of nitro-compound.
Furthermore, in order to inhibit the production of undesirable impurities as by-products, it is suggested in the above-mentioned Japanese Patent Publication No. 50-15779 that an organic base such as alkanolamine be added. For example, in the case of the preparation of aniline by hydrogenation of nitrobenzene, triethanolamine is added but, even in this case, condensed aniline which has been distilled off still contains about 0.02% or less of nitrobenzene and about 0.5% or less of impurities having hydrogenated nuclei. It is further described in the above-mentioned publication that when triethanolamine is not added, a larger amount of impurities is present in the condensed aniline and it is difficult to separate the aniline layer of the condensed aniline product from a water layer. In fact, when the present inventors carried out a tracing test by the use of a diatomaceous earth-nickel catalyst in accordance with the procedure of Example 1 in Japanese Patent Publication No. 50-15779, the condensed aniline which was distilled off contained 0.05% or more of unreacted nitrobenzene and 0.6% or more of compounds having hydrogenated nuclei.
As methods for eliminating these drawbacks, the present inventors have filed two patent applications regarding processes for preparing high-purity aniline. The first process comprises the steps of dispersing, in an aniline solvent, a lipophilic carbon having an oil absorbency of at least 100 on which a catalyst of palladium or palladium-platinum is deposited, and then carrying out the reaction at a temperature of from 150.degree. to 250.degree. C. substantially in the absence of water while the concentration of nitrobenzene in the reaction solution is maintained at 0.5% by weight or less and during which, aniline and water formed in the reaction are continuously distilled out as vapor from the reaction product (Japanese Patent Laid-open No. 57-1679460). The second process for continuously preparing high-purity aniline by the catalytic hydrogenation of nitrobenzene comprises the steps of carrying out the reaction in the presence of a compound selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, alkali metal bicarbonates, zinc acetate and zinc nitrate during which the concentration of nitrobenzene in the aniline reaction solution is maintained at 0.5% by weight or less (Japanese Patent Laid-open No. 58-4750). These methods can provide aniline having a higher purity than the conventional process, but further improvement is desired.